Overview
Within the second Strategic Highway Research Program (SHRP 2), Project L02 focused on creating a suite of methods by which transportation agencies could monitor and evaluate travel time reliability. Creation of the methods also produced an improved understanding of why and how travel times vary and the factors that create that variation.
This final report provides a brief narrative about what reliability is and how it can be measured and analyzed. A general finding is that reliability is best described by creating holistic pictures like probability density functions (PDFs) and their associated cumulative density functions (CDFs). PDFs are helpful for identifying multimodality or the existence of multiple operating conditions within the data being examined (Barkley et al. 2012; Guo et al. 2010; Fraley and Raftery 2009). CDFs are helpful for seeing if progress is being made in making a system more reliable or for comparing the reliability of one system against another.
A survey of the state of the art and state of the practice in travel time reliability monitoring systems (TTRMSs) worldwide guided the development of the methods. The survey showed that Europe and Asia were slightly ahead of the United States at the time the project started. A second survey among potential future users of the monitoring system guided its functional features. The potential users included various groups: (1) system administrators and their staffs, (2) highway system operators, (3) transit system operators, (4) freight service providers, (5) highway system users, (6) transit system users, and (7) freight system users. Each group of users had its own needs, with consistency being evident among the system operators (Groups 1, 2, 3, and 4) and system users (Groups 5, 6, and 7). The findings from the second survey were coalesced into a set of use cases that became the driving force behind the system’s functional specifications.
One of the project’s main products is the L02 Guide to Establishing Monitoring Programs for Travel Time Reliability, which describes how an agency should develop and use a TTRMS. The monitoring system is not intended to stand alone. Rather, it is intended to work with an existing traffic management system. The Guide follows the block diagram presented in Figure ES.1 to describe a TTRMS. Each module is shown as a box, and the inputs and outputs are shown as circles.
The three major modules of the monitoring system are a data manager, a computational engine, and a report generator. The data manager assembles incoming information from traffic sensors and other systems, such as weather data feeds and incident reporting systems, and places them in a database that is ready for analysis as “cleaned data.” The computational engine works off the cleaned data to prepare pictures of the system’s reliability: when it is reliable, when it is not, to what extent, under what conditions, and so forth.